Despite the dramatic success of highly active antiretroviral therapy (HAART) in inhibiting viral replication in HIV-infected subjects, it is increasingly clear that there is a compelling need for the development of complementary therapies. Accumulating experience with HAART is documenting a significant incidence of serious side effects, an increasing prevalence of resistant viruses, and failure rates of HAART that exceed 50 percent in some cohorts. A wide range of genetic strategies are now able to achieve potent inhibition of HIV replication in vitro, and introduction of these inhibitory genes into hematopoietic stem cells offers the potential to offer long- lived immune reconstitution. However, the successful translation of these in vitro successes to clinically- applicable therapies has been limited by the disappointing rates of gene transfer to hematopoietic cells in human gene therapy clinical trials. Multiple ethical and practical considerations significantly constrain the ability to address basic questions regarding stem cell gene therapy for AIDS in human clinical trials. Experiments in nonhuman primates offer the opportunity to rigorously address these issues in an in vivo experimental model. Experiments conducted during the initial funding period of this grant have shown that significant levels of genetically-modified cells can be obtained in nonhuman primates following autologous bone marrow transplantation with hematopoietic stem cells transduced with murine leukemia virus (MLV) vectors. Specific aims of the current proposal are: 1. To optimize MLV and lentiviral vectors for the delivery of RNA decoys into hematopoietic stem cells and evaluate their ability to inhibit SIV/HIV replication; 2. To examine levels of gene marking in uninfected macaques transplanted with hematopoietic stem cells transduced with MLV and lentiviral vectors; 3. To determine the ability of inhibitory genes to protect hematopoietic cells from SHIV infection in vivo; and 4. To examine the ability of genetically-modified hematopoietic stem cells to reconstitute immune function in SHIV-infected macaques. These studies should yield important information regarding the efficacy and safety of stem cell gene therapy for AIDS and facilitate the development of similar trials in HIV- infected people.